This invention is in the field of methods and apparatus for heat treatment operations. More particularly, the invention relates to batch coil annealing furnaces and will be described with particular reference thereto. However, it will be appreciated by those skilled in the art that the invention may generally be applied to heat treating operations where at least one work item having an axial passage is placed within an enclosure in a heat transfer relationship with a heating and cooling media within the enclosure.
Annealing of metal strips and the like is generally accomplished by winding the strips into coils having an axial passage bounded by the inner diameter of the winding. Several coils can be stacked on top of one another and are sealingly enclosed in an inner cover. The inner cover is enclosed in an outer furnace chamber. This may be accomplished in single-stand or multi-stand batch coil annealing furnaces. Heat is transferred through the outer furnace chamber to heat the inner covers which in turn transfer the heat to the coils. A proper annealing atmosphere is maintained in the inner covers. The primary mode of heat transfer from the cover to the coils is by radiation. Additionally, the atmosphere is circulated within the inner cover to achieve more rapid and uniform heat transfer by convection.
Coils are stacked coaxially upon one another within the cover with the axial passage of each coil aligned to form an axial path. A radial fan in the base of the furnace is aligned with the axial path and forces the inner cover atmosphere radially away from the center of the cover, through a base space which communicates from the radial fan, to the annular space between the stack of coils and the inner cover wall. The atmosphere passes up through a top space between the top of the coils and the top of the inner cover and back down to the fan through the axial path in the center of the stack of coils.
Even with the use of the radial fan, there is non-uniform heat transfer and the rate of annealing is limited. The atmosphere heats as it rises in the annular space between the coils and inner cover and is hottest when it reaches the top of the stack of coils. The top outside corner of the top coil is exposed to the radiant energy from the side and the top of the inner cover and is the hottest spot in the stack of coils. The hot atmosphere is forced down through the axial path and cools as it descends to the fan. The top coil, therefore, sees an unequal and greater amount of heat than the lower coils, as one moves progressively down a stack. This problem is compounded by the fact that the upper coils in the stack are usually the smaller and lightest coils.
The supports used in the base of batch coil annealing furnaces, currently in use with radial fans, must be strategically located within the base and aerodynamically designed. This design is necessary to minimize pressure drop and disturbance of the flow pattern of the wind in the immediate area of the fan.
Radial fans now in use are usually 24 inch O.D., with a motor capacity of 25 horsepower and capable of flows of about 5,000 to 10,000 standard cubic feet per minute. Even if radial fans are modified to operate at higher flow rates, a hot spot would develop in the upper coils for the reasons discussed above. There is a need in the art for a method of providing increased heating rates with uniform heat transfer to work items within the inner cover being heated in a furnace and, more particularly, a more rapid uniform method of heat transfer within the cover of a batch coil annealing furnace.